Dickeya dadantii is a pectinolytic phytopathogen enterobacterium that causes soft rot disease on a wide range of plant species. The virulence of D. dadantii involves several factors, including the osmoregulated periplasmic glucans (OPGs) that are general constituents of the envelope of proteobacteria. In addition to the loss of virulence, opg-negative mutants display a pleiotropic phenotype, including decreased motility and increased exopolysaccharide synthesis. A nitrosoguanidine-induced mutagenesis was performed on the opgG strain, and restoration of motility was used as a screen. The phenotype of the opg mutant echoes that of the Rcs system: high level activation of the RcsCD-RcsB phosphorelay is needed to activate exopolysaccharide synthesis and to repress motility, while low level activation is required for virulence in enterobacteria. Here, we show that mutations in the RcsCDB phosphorelay system restored virulence and motility in a D. dadantii opg-negative strain, indicating a relationship between the Rcs phosphorelay and OPGs.Osmoregulated periplasmic glucans (OPGs) are general periplasmic constituents of the envelope of most proteobacteria. Their common features are that glucose is the sole constituent sugar, and their abundance in the periplasm increases as the osmolarity of the medium decreases. In Enterobacteriaceae and related bacteria, the glucose backbone synthesis is catalyzed by both products of the opgGH operon (5). Studies of several bacterial pathogens, including Dickeya dadantii, showed the importance of OPGs for virulence (4,5,18,25,26).Dickeya dadantii is a member of the pectinolytic erwiniae causing soft rot disease in a wide range of plant species (33). The virulence of D. dadantii is associated with the synthesis and the secretion of a set of plant cell wall-degrading enzymes (pectinases, cellulases, and proteases) causing maceration of the plant tissues (22). D. dadantii synthesize OPGs containing 5 to 12 glucose units joined by ␤,1-2 linkages and branched by ␤,1-6 linkages that are substituted with succinyl and acetyl residues (11). The opgG or opgH mutants unable to synthesize OPGs show a pleiotropic phenotype. They are nonvirulent on chicory leaves and potato tubers, and synthesis and secretion of pectate-lyases, cellulases, and proteases are reduced (32). Motility is severely reduced, while exopolysaccharide secretion is increased (mucoid phenotype) (32). Data suggest that the opg mutants are impaired in perception of the environment, which prevents D. dadantii from recognizing host cells, suggesting a possible dysfunction of phosphorelay signaling pathways, major systems required for environmental perception in bacteria (6). In these systems, upon stimuli, a kinase/phosphatase sensor autophosphorylates and transfers the phosphate group to a cytoplasmic regulator which modulates expression of target genes.Here, we show that mutations in the rcsC and rcsB genes, encoding, respectively, the sensor and the cognate regulator of the RcsCD-RcsB phosphorelay, suppress several phenotypes o...

Osmoregulated periplasmic glucans (OPGs) are general constituents of the envelope of Gram-negative bacteria. They are required for full virulence of bacterial phytopathogens such as Pseudomonas syringae, Xanthomonas campestris and Erwinia chrysanthemi. E. chrysanthemi is a pectinolytic c-proteobacterium that causes soft rot disease on a wide range of plant species. In addition to the loss of virulence, opg mutants exhibit a pleiotropic phenotype that affects motility, bile-salt resistance, exoenzyme secretion, exopolysaccharide synthesis and membrane lipid composition. This is believed to be the first proteomic analysis of an OPG-defective mutant of E. chrysanthemi and it revealed that, in addition to the effects described, catabolic enzyme synthesis was enhanced and there was a greater abundance of some proteins catalysing the folding and degradation of proteins needed for various stress responses. Thus, in the opg mutant strain, loss of virulence was the result of a combination of envelope structure changes and cellular metabolism modifications.

No abstract